In semiconductor manufacturing, electrostatic clamps (also called electrostatic chucks or ESCs) are often utilized to hold a workpiece (e.g., a semiconductor wafer) in position while the workpiece undergoes various semiconductor processing, such as ion implantation. During such semiconductor processing, it is often desirable to accurately maintain the position of the workpiece with respect to the electrostatic clamp and/or maintain a temperature of the workpiece via backside cooling of the workpiece through the electrostatic clamp. Maintaining the position and/or temperature of the workpiece often requires the workpiece to maintain a predetermined contact pressure with a surface of the electrostatic clamp.
In order to maintain the position and/or predetermined contact pressure of the workpiece with respect to the electrostatic clamp, tests are often performed to ensure proper clamping forces are maintained throughout clamping and processing of the workpiece. Such tests can include workpiece lifting tests, wherein a force required to remove the workpiece from the electrostatic clamp is measured by a load cell coupled to a test workpiece. The force required to remove the workpiece is then utilized to determine an ability of the electrostatic clamp to maintain the clamping of the workpiece.
Another test for determining clamping forces attained by the electrostatic clamp is often utilized when backside gases are used for cooling of the workpiece during processing (a so-called “pop-off” test). Such backside gases are provided to a backside of the workpiece via orifices in the surface of the electrostatic clamp, wherein a pressure and/or flow of the backside gases at least partially counteracts the clamping force applied to the workpiece by the electrostatic clamp. The pressure or flow of the backside gas is increased to a level in which the workpiece separates from the electrostatic clamp, therein providing a measure of the clamping force associated with the separation, and thus, a condition of the clamping capability of the electrostatic clamp.
It is noted, however, that such clamping tests are often interruptive in a production environment, wherein specialized workpieces and/or devices are inserted into the processing equipment, thus expending valuable production time. Further, since the tests are intermittently performed (e.g., one test is performed between many workpiece clamping cycles), a quick deterioration of the clamping capability of the electrostatic clamp may not be readily recognized and corrected for.
Furthermore, once the conventional clamping tests are performed, a set of electrostatic clamping parameters (e.g., a voltage and frequency applied to the electrostatic clamp) are utilized for subsequent clamping, wherein the clamping parameters are not modified until a subsequent clamping test is performed. The clamping pressure generated in the electrostatic clamp can vary for many reasons, however, such as the age of the electrostatic clamp, coatings on the surface of the ESC, contamination present on the surface of the electrostatic clamp, etc. Heretofore, such variations of clamping pressure have not been adequately accounted for in real time, concurrent with the clamping of the workpiece.